Systems exist that use a plurality of sensors to collect data and to use that data in connection with sensing, monitoring and control functionality. Aircraft monitoring systems may use a plurality of sensors that provide sensed data to one or more central processors that use the data to assess the health of system components. For example, U.S. Pat. No. 6,847,917 to Bechhoefer, which is incorporated by reference herein, discloses determining a health indicator of a component using condition indicators (CIs) to parameterize characteristics about a component. The CIs are based on sensed data for the component and facilitate condition-based (rather than schedule-based) maintenance.
Wireless communication is being considered as a replacement for wired data transfer, particularly in avionics systems. Potential advantages of using wireless communication as opposed to wired connections include lower weight, reduced design complexity, easier system reconfiguration, improved diagnostic capabilities, simplified installation and/or easier maintenance. For example, retrofitting new sub-systems into existing aircraft may become substantially easier and cheaper if no new wiring has to be designed and installed, and the use of a large number of sensors to monitor an aircraft's state may become more realistic if it does not lead to an increased amount of wiring. Further, wireless communication may enable the introduction of new sensing technologies into new aircraft design and/or existing aircraft as retrofit options. For the wireless approach to become practically viable, however, it must deliver a level of performance comparable to wired systems, and stringent performance requirements may need to be satisfied. Aircraft applications typically require very high reliability, i.e. data loss has to be very infrequent. The required probability of lost data may be several orders of magnitude lower than that typically considered acceptable by vendors of commercial wireless components. In addition, data latency requirements may be quite strict and a high precision of synchronization may be required.
Synchronization issues arise when multiple devices need to perform the same action at the same time. For example, mechanical diagnostics may involve collection of vibration data from multiple accelerometers. The difference in phase between the vibration signals may be used, among other features, to infer information about the mechanical health of the aircraft. In order to extract phase information, it may therefore be necessary that the acceleration data samples be acquired from different sensors starting at the same time. This may not be achieved, however, by simultaneously commanding all sensor nodes to immediately acquire data since wireless communication can be subject to unpredictable packet losses that may necessitate retransmissions to some of the nodes. Also, transfer of information to and from transceiver modules may be subject to unpredictable delays. Because of this, the same data acquisition command broadcasted to several destinations may reach the sensors at different time instances. This, in turn, may result in different sensors starting data acquisition at different times, leading to incorrect mechanical health information. Accordingly, to be effective, a wireless communication system must address synchronization issues among sensors.
Known wireless transfer systems include synchronization mechanisms internal to their communication protocols. For example, in the IEEE 802.11 standard, the time synchronization function (TSF) provides synchronization in connection with beacon messages transmitted periodically by a single node. However, the internal synchronization information of the wireless transceiver may not be available to an attached device, such as a sensor device, and thus synchronization among multiple sensor devices is still performed through the exchange of time data among the sensors, which remains subject to unpredictable and random transmission delays. For a discussion of known synchronization techniques among wireless devices, see Holger Karl and Andreas Willig, “Protocols and architectures for wireless sensor networks,” Wiley, 2005.
A drawback of known synchronization techniques is the need to send synchronization messages that are separate from the messages used for data transfer. The sensor radio components need to be activated for synchronization. However, in some cases, the radio components consume a relatively large amount of energy of a wireless sensor. In addition, initializing the radio components may require a significant amount of time, thus further exacerbating energy used for sensor synchronization. Thus, activating and operating the wireless sensors just for synchronization reduces the battery life of the sensors.
Accordingly, it is desirable to provide a wireless communication system that may be suitably used in connection with sensing, monitoring and control systems, such as avionics systems, that can be synchronized in a way the preserves battery life of the wireless sensors.